[Amps] Homebrew "Hi-Pot" design issues

[Ian White, G3SEK]

Hal wrote:
>
>http://www.discovercircuits.com/F/flyfack.htm
>
>This is a website that discusses the "25 ~ 30 KV Power Supply."
>
>With such a tiny amperage requirement the regulation and control of the 
>primary circuitry is cheap and simple.
>

The link to http://www.powerlabs.org/flybackdriver.htm  looks good too. 
It's a simple 1-transistor oscillator using extra primary and feedback 
windings on the existing flyback transformer core.

I haven't tried this particular HB circuit, but have had good results 
with a very similar oscillator/transformer/rectifier in a packaged unit. 
It uses a single 2N3053 oscillator, and has to be the same kind of 
circuit as the one on the web page.

By varying the DC supply voltage, you can get a useful range of high 
voltages.

The lower limit on output voltage is where the DC input voltage is so 
low that the oscillator stops. The upper limit where something arcs over 
or burns out!  For practical hi-pot testing, you don't need a lower 
voltage than the HV supply in your amp, so it's not a problem if the 
oscillator drops out anywhere below about 2kV. Also we don't need such 
high voltages as those guys who play with big sparks, so we can probably 
use more primary and secondary turns, and go easy on the DC input 
voltage.

My DC supply circuit is a small 15-20V 1A mains supply feeding a 
standard LM317T 1-amp variable voltage regulator (straight from the data 
sheet) with a front-panel control. The internal current limiting in the 
LM317 means that nothing will burn out if the oscillator has stalled.

Because we don't need to know the output voltage very accurately for 
hi-pot testing, I didn't use a permanent voltmeter. Instead I borrowed 
an electrostatic voltmeter, and calibrated the front-panel DC voltage 
control directly in terms of kV output voltage. A sensitive leakage 
current meter with protection diodes finished the job.

A hi-pot tester is yet another of those gadgets that you don't need all 
the time... but sometimes it comes in very handy indeed.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek